Turning waste heat into steam

Paul Boughton

It seems that there is always scope to increase the efficiency of the internal combustion engine by small increments, yet there remains a desire to find the next step change in efficiency for automotive powerplants.

With the internal combustion engine already so highly developed, resources are therefore being invested in alternative technologies, such as fuel cells and hybrids.

There is no escaping the fact that burning a fuel/air mixture in an internal combustion engine produces mechanical energy and heat, so considerable engineering effort is devoted to managing the heat in new vehicle developments. Not only does this involve the engine’s cooling system, but also the underbonnet (underhood) systems and the exhaust. The heat is largely considered to be a ‘problem’ – or at least the cause of problems that must be resolved.

The northern region of South Africa seems to be a good place for innovative ideas – especially when it comes to heat. Professor Dr Raymond Freymann, head of BMW’s research and technology division, once tested the heat resistance of trial engines in the Kalahari Desert, where temperatures in the shade exceed 50°C.

Professor Freymann explains why engine heat is such an issue: “This is because two-thirds of the energy contained in petrol is lost via exhaust emissions and cooling water, and the heat in the cooling water has to be disposed of via the radiator.”

In the heat of the African desert, however, engineers from BMW’s research and technology division came upon the brilliant idea of tapping into all this unused energy. The resulting system is astoundingly logical and is essentially centuries old.

The engineers believe that by taking a ‘combined heat and power’ (CHP) approach, efficiency improvements of up to 15percent may be feasible in a mid-range car such as the 3series BMW (Fig.1).

Combined heat and power, also sometimes called cogeneration or total energy, is not uncommon today, but the concept is usually applied in, for example, leisure facilities, hotels and communities where a prime mover is used to generate both electrical power and heat.

The result is an overall improvement in efficiency – and operating costs – compared with the existing conventional means of providing electricity and heating. But applying the combined heat and power concept to a vehicle is novel.

Efficiency improvements

According to Dr Andreas Obieglo, the project leader, combining an innovative assistance drive with a 1.8litre BMW four-cylinder engine reduced fuel consumption by up to 15percent and generated 10kW (13.6PS) more power and up to 20Nm more torque.

In an optimal scenario, travelling at a constant speed of 120km/h, a car with an average consumption of 7litres of fuel per 100km could save 1.5litres per 100kilometres. Moreover, the system can also be optimised for use in urban traffic.
All of this is a considerable improvement over the small performance enhancements that are achieved by most other engine development programmes today.

The reason behind the larger-than-usual pay-back is that ‘waste’ energy is being recovered from the heat present in the exhaust gases and cooling water, so no additional fuel is required.

BMW says that the research project therefore meets all the conditions espoused by its philosophy of ‘efficient dynamics’ – lower emissions and fuel consumption combined with the creation of more dynamic driving and performance characteristics.

Same principles as steam engine

Now known as the Turbosteamer project, the team is applying some of the same principles as used in the steam engine. The Turbosteamer is based on two circuits: “The first and most important is the ‘high-temperature circuit’, which uses the exhaust heat of petrol-driven cars as an energy source,” explains Dr Obieglo.

In addition there are two heat exchangers in the exhaust system through which water is pumped at a pressure of up to 40 bar. Even if the engine is only placed under moderate load, the water in this circuit is heated up to a maximum of 55°C.

Exhaust gases

More than 80percent of the heat energy contained in the exhaust gases is recycled using this technology.

The hot steam flows into an expansion unit that is coupled to the crankshaft. There the highly-pressurised steam is converted into useful energy and added to the powertrain.
Once the steam has done its work, it cools and condenses back into water, ready to continue around the circuit.

The second circuit in the Turbosteamer system works with lower temperatures. In this closed-loop system, however, it is not water that circulates, but ethanol.

This is heated up to 150¢ªC, both by the cooling water carrying heat from the engine and by heat from the high-temperature circuit.

“In this way we ensure sufficient energy for heating up the ethanol to approximately 100degrees in order to drive a second expansion unit that also contributes to a performance increase or fuel consumption decrease,” explains Dr Obieglo.

While the temperature was found to be sufficient to operate the second expansion unit, the efficiency was not yet satisfactory.
“For this reason we installed a second loop at the end of the exhaust heat exchanger, a point at which enough heat is also released to heat up the ethanol to approximately 150degrees,” says Dr Obieglo.

The second expansion unit converts this heat into additional rotation, and the low-temperature circuit is completed by a conventional radiator that releases the remaining warmth from the ethanol’s condensation into the surrounding atmosphere (Fig.2).

Fit for the future

Professor Burkhard Göschel, a member of the board of management responsible for development and purchasing at BMW, comments: “The Turbosteamer reinforces our confidence that the internal combustion engine is undoubtedly a technology fit for the future.”

The development of the assistance drive has now reached a stage where it is undergoing comprehensive trials on a test rig (Fig.3). Components for this drive have been designed so that they are capable of being installed in existing models.

Tests have been carried out on a number of sample packages to ensure that a car such as the BMW3series provides adequate space.
Engine compartments of four-cylinder models offer enough space to allow the expansion units to be accommodated.

Dr Obieglo’s team has already been working for more than five years on the technology.
“In around 10 years’ time the Turbosteamer could be ready for mass production,” forecasts Dr Obieglo. Until then the engineers will be working on ways to significantly reduce the weight of the system from the current 50kg per circuit.

Furthermore, the components need to be made smaller and the efficiency of the expansion units improved (Fig.4).
In order to realise these aims, the company is now seeking co-operative partnerships with car component suppliers.

While the concept has the greatest impact when used with a petrol internal combustion engine, it is also suitable for use with diesel engines, with no alteration work necessary.

“In the case of diesel engines, however, the exhaust emissions are lower and, consequently, so is the amount of energy that can be regained,” explains Dr Obieglo.
BMW Group Research and Engineering has demonstrated the medium-term perspectives of the company’s ‘efficient dynamics’ philosophy.

“This project resolves the apparent contradiction between consumption and emission reductions on the one hand, and performance and agility on the other,” says Professor Göschel.

Reduction in consumption

The BMW Group is committed to the principle that a reduction in consumption amounting to a few per centage points over the entire model range exerts a greater overall effect on the general population than more per centage points for a niche model (such as a hybrid vehicle, for example).

BMW is therefore focusing on making the latest technologies for reduced fuel consumption accessible to as many people as possible.

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